1. Technical Field
The invention relates to an optical switch comprising at least one first fluid and a second fluid immiscible with each other within a space between a first transparent support plate and a second support plate, the second fluid being electroconductive or polar.
In particular the invention relates to a display device comprising picture elements (pixels) having at least one first fluid and a second fluid immiscible with each other within a space between a first transparent support plate and a second support plate, the second fluid being electroconductive or polar.
2. Related Art
Optical switches may be used in shutter applications, diaphragms, but also in switchable color filters in e.g. display applications.
Display devices like TFT-LCDs are used in laptop computers and in organizers, but also find an increasingly wider application in GSM telephones. Instead of LCDs, for example, (polymer) LED display devices are also being used.
Apart from these display effects that are well established by now other display techniques are evolving like electrophoretic displays, which are suitable for paper white applications.
The invention is based on a principle called electro-wetting. The invention provides new ways of using this principle in which one of the fluids in a first state adjoins a greater part of the first support plate and in the second state the other fluid at least partly adjoins the first support plate.
If for instance a (first) fluid is a (colored) oil and the second (the other) fluid is water (due to interfacial tensions) a two layer system is provided which comprises a water layer and an oil layer. However, if a voltage is applied between the water and an electrode on the first support plate the oil layer moves aside or breaks up due to electrostatic forces. Since parts of the water now penetrate the oil layer the picture element becomes partly transparent.
Display devices based on this principle have been described in PCT-Application WO 03/071346. In most of the embodiments shown in this Application, a picture element corresponds to a defined space (e.g., a substantially closed space) and, in the second state, the other fluid substantially completely adjoins the first support plate. In one embodiment shown in this Application, picture elements are described in which the pixel walls do not extend across the whole pixel thickness.
In practice however these pixel walls have a certain wall height of about 80 μm or more which has severe implications for the optical performance of small pixels in particular. The presence of such pixel walls results in an effectively reduced pixel area, since the area close to the walls does not contribute to the pixel brightness at the higher incoming angles. As a result, the total pixel will become less bright. This reduction of brightness will be more severe for smaller pixels. At a wall height of about 80 μm, the loss of brightness is about 10-15% for a 500 μm×500 μm pixel.
Moreover the walls are generally glued to the underlying surface. This underlying surface is often a fluoropolymer surface so the bonding is rather weak mechanically due to the fact that the fluoropolymer is very hydrophobic. Also the glue in between the walls and the fluoropolymer surface tends to move into the pixel area. In this case, the oil will not cover this part of the pixel, since the oil is repelled from the glue surface. These areas cause a considerable increase in reflectivity in the “black” off-state, thereby reducing the optical contrast.